Flame Inhibition
Flameproofing agents are fire inhibitors which are intended to restrict, slow down or prevent the spread of fires.
Flameproofing agents are used wherever potential sources of ignition are located, or where the use of combustible materials constitutes a safety risk.
Rising demands for safety and the increasing use of high-grade plastics instead of metals and metal alloys, for example in the construction industry, aircraft and automobile manufacture and in interior fittings, have led to an increasing need for flameproofing agents.
The mode of action of flameproofing agents is based on different effects:                interruption of the radical chain reaction of the gases produced during pyrolysis of the material;        forming a protective coating of charred material (intumescence), in order to prevent the access of oxygen and heat;        cooling of the combustion process by initiation of an endothermic decomposition or evaporation of bound water;        dilution of the combustible gases by inert, gaseous substances (such as for example CO2 which is produced by endothermic decomposition of carbonates;        liquefaction, i.e. formation of a melt which flows out of the fire zone and simultaneously reduces the surface area.        
Most flameproofing agents trigger one or more of the said chemical-physical effects:
Therefore the following 4 types of flameproofing agent are distinguished:                additive flameproofing agents—these are incorporated into the combustible substances;        reactive flameproofing agents—substances which are themselves components of the material by polymerization into plastic;        inherent flameproofing agents—the material per se is flame-resistant;        coating—the flameproofing agent is applied as a coating onto the combustible substance from the outside.        
In this case in reactive flameproofing agents there are three different main approaches in order to achieve a certain flameproofing in polymer systems:                Intumescent FR systems (English: flame retardant): For example melamine derivatives with polymer;        Halogen-free FR systems: In particular aluminum hydroxide (ATH), magnesium hydroxide (MDH), ammonium polyphosphate (APP); and/or        Halogen-containing FR systems: For example polyvinyl chloride (PVC) with antimony trioxide (Sb203) and/or organic halogen FR with antimony trioxide (Sb203) and polymer.        
Important reactive and inherent, but also additive flameproofing agents have been criticized for toxicological reasons, in particular because of the formation of toxic or environmentally harmful gases during the decomposition process, and are now subject to a strict risk assessment. This applies in particular to halogen-containing flameproofing agents, so that there is a great demand for so-called zero-halogen flame retardants (OHFR fillers) and in particular inorganic flameproofing agents have increasingly gained importance and continue to do so.
Aluminum hydroxide (ATH) is the most important inorganic flameproofing in terms of quantity. Initially ATH was used in polyolefin systems, in particular for so-called wire and cable (W&C) applications.
Since the nineteen-seventies alternatives to these have been increasingly tested, such as for example magnesium hydroxide (MDH), which has a higher decomposition temperature and also has been and is used in systems such as polypropylene and polyamides as OHFR filler. Disadvantages of MDH are, on the one hand, the comparatively high price thereof and, on the other hand, problems which occur at processing temperatures above 300° C., since here the products on the market already decompose while undergoing elimination of water.
At the same time as ATH, ammonium polyphosphate (APP) and its derivatives occurred, but these have inter alia the disadvantage that they are susceptible to moisture content (water immersion tests in electrical components) and already release ammonia at a temperature of 170° C.
In the technical thermoplastics, such as for example polyamides and polyesters, melamine derivatives such as melamine isocyanurate, melamine cyanurate and melamine polyphosphate have been used in addition to magnesium hydroxide.
Furthermore at load factors of up to 65% by weight of the respective OHFR filler in the flameproof polymer compound, which may be necessary for example in order to achieve a required fire class (for example UL 94 vertical V-0), technical problems occur with regard to processing (extrusion behavior) as well as property profile problems (decrease in the mechanical and electrical values by comparison with the unfilled polymer). These difficulties increase at even higher load factors—virtually exponentially—and make maximum demands on compounding equipment and operators. This statement applies to an even greater extent for the so-called Construction Protection Directive (“CPD”) (cf. prEN 50399), which for classification of the FR compounds into the so-called “Euro Classes” again necessitates more stringent applications of new FR tests. At such high degrees of filling, flameproof polymer compounds can no longer be technically implemented in practice and can no longer be processed. Furthermore, during the development of CPD-compliant OHFR polymer compounds the use of so-called nanoclays as synergists has been described and for example introduced technically into the market under the trade marks Cloisite® and Nanofil®.
Since the polymer compound systems resulting therefrom have become increasingly complex and expensive, the inventors of the present invention have taken a completely different route.
It is known that red mud, which is produced as a waste product in the Bayer process for extracting aluminum hydroxide (ATH) from bauxite, has profound OHFR characteristics. Therefore in the following description red mud (RM) is understood to be the residue from the Bayer process which is produced in the extraction of ATH from bauxite.
Red mud (RM), which may to some extent be represented as bauxite minus ATH, is an extremely heterogeneous substance with regard to its chemical and mineralogical composition, its endothermic properties, its pH value, etc. The cause of the heterogeneity sometimes lies in the differing composition the bauxites used, but above all in whether the Bayer process operates by autoclave digestion or by tube digestion. In the autoclave process the digestion is carried out with 30 to 35% caustic soda solution at temperatures of 170-180° C., so that a pressure of 6 to 8 bars is established. The tube digestion process was developed in order to shorten the reaction time of 6 to 8 hours to less than 1 hour by increasing the temperature to 270° C. However, at this temperature a water vapor pressure of 60 bars is established at the end of the reactor. The higher temperatures of the tube digestion also influence the composition of the red mud. For example, in the iron hydroxide/oxide hydroxide system in the tube digestion process the balance is shifted almost completely towards hematite (Fe2O3). Because of the heterogeneity of the red mud (RM) the economically viable possibilities for use thereof is restricted, so that it must be predominantly disposed of as waste at disposal sites.
In WO 2012/126487 A1 an OHFR-system based upon modified rehydrated red mud (MR2S) is described, which is suitable as a cost-effective OHFR system for technical applications in the wire and cable field or for constructional and plastics processing applications. With the aid of the modified rehydrated red mud disclosed in WO 2012/126487 A1 a flame-retardant effect can be achieved in the temperature range from approximately 200° C.-350° C. The flame-retardant effect comes about due to the fact that the hydroxides and oxide hydroxides of aluminum and iron—such as for example gibbsite and boehmite or goethite—which are produced in the rehydration of the red mud decompose in oxides and water. Such products have applications for example in polymer systems such as PVC or EVA (PE). Products such as ATH or APP hitherto used in the market react between 180° C. and 220° C. and are regarded as low-temperature products. Between 220° C. and 340° C. products such as MDH and brucite are used which are regarded as high-temperature products. The flame retardants (MR2S) produced from RM by rehydration react between approximately 220° C. and 350° C. and thus according to the currently customary definition covers both the low-temperature and the high-temperature range.
However, within the context of the present disclosure the temperature range between 220° C. and 350° C. is classified as the low-temperature range, MR2S acquires the suffix NT (low temperature) and thus is called MR2S-NT.
However, there is an increasing need for substances which exhibit their flame-retardant effect in higher temperature ranges, so-called high-temperature (HT) flame retardants. Within the context of the present disclosure the temperature range between 350° C. and 500° C. is classified as the high-temperature range.